1. Field of the Invention
The present invention relates to an optical image measuring apparatus that applies a light beam to an object to be measured, particularly a light scattering medium, and produces a surface form or inner form of the object to be measured by detecting a reflected light beam or a transmitted light beam. In particular, the present invention relates to an optical image measuring apparatus for measuring the surface form or inner form of the object to be measured by using an optical heterodyne detection method to produce the image of the measured form.
2. Description of the Related Art
In recent years, attention has been given to optical imaging technique that produces an image of a surface or inner portion of an object to be measured using a laser light source or the like. This optical imaging technique is not hazardous to human bodies in contrast to the conventional X-ray CT. Therefore, the development of applications in the medical field has been particularly expected.
An example of a typical method of the optical imaging technique is a low coherent interference method (also called ‘optical coherence tomography’ or the like). This method uses the low coherence of a broad-band light source having a broad spectral width, such as a super luminescent diode (SLD). According to this method, reflection light from an object to be measured or light transmitted therethrough can be detected at superior distance resolution on the order of μm (for example, see Naohiro Tanno, “Kogaku” (Japanese Journal of Optics), Volume 28, No. 3, 116 (1999)).
FIG. 7 shows a basic structure of a conventional optical image measuring apparatus based on a Michelson interferometer, as an example of an apparatus using the low coherent interference method. An optical image measuring apparatus 300 includes a broad-band light source 301, a mirror 302, a beam splitter 303, and a photo detector 304. An object to be measured 305 is made of a scattering medium. A light beam from the broad-band light source 301 is divided by the beam splitter 303 into two, that is, reference light R propagating to the mirror 302 and signal light S propagating to the object to be measured 305. The reference light R is light reflected by the beam splitter 303. The signal light S is light transmitted through the beam splitter 303.
Here, as shown in FIG. 7, a propagating direction of the signal light S is set as a z-axis direction and a plane orthogonal to the propagating direction of the signal light S is defined as an x-y plane. The mirror 302 is movable in a direction indicated by a double-headed arrow in FIG. 7 (z-scanning direction).
The reference light R is subjected to a Doppler frequency shift through when reflected by the z-scanning mirror 302. On the other hand, the signal light S is reflected from the surface of the object to be measured 305 and from the inner layers thereof when the object to be measured 305 is irradiated with the signal light S. The object to be measured 305 is made of the scattering medium, so reflection light of the signal light S may be a diffusing wave having random phases. The signal light propagating through the object to be measured 305 and the reference light that propagates through the mirror 302 to be subjected to the frequency shift are superimposed on each other by the beam splitter 303 to produce interference light.
In the image measurement using such a low coherent interference method, interference occurs only when a difference in optical path length between the signal light S and the reference light R is within the coherence length (coherent distance) on the order of μm of the light source. In addition, only the component of the signal light S whose phase is correlated to that of the reference light R interferes with the reference light R. That is, only the coherent signal light component of the signal light S selectively interferes with the reference light R. Based on their principles, the position of the mirror 302 is shifted by the z-scanning to vary the optical path length of the reference light R, so that a reflectance profile of the inner layers of the object to be measured 305 is measured. The object to be measured 305 is also scanned with the irradiated signal lights in an x-y plane direction. The interference light is detected by the photo detector 304 during such scanning in the z-direction and the x-y plane direction. An electrical signal (heterodyne signal) outputted as a detection result is analyzed to obtain a two-dimensional sectional image of the object to be measured 305 (see Naohiro Tanno, “Kogaku” (Japanese Journal of Optics), Volume 28, No. 3, 116 (1999)).
Assume that an intensity of the reference light R and an intensity of the signal light S which are superimposed by the beam splitter 303 are given by Ir and Is, respectively, and a frequency difference between the reference light R and the signal light S and a phase difference therebetween are given by fif and Δθ, respectively. In this case, a heterodyne signal as expressed by the following expression is outputted from the photo detector (for example, see Yoshizawa and Seta “Optical Heterodyne Technology (revised edition)”, New Technology Communications (2003), p. 2).
Expression (1)i(t)∝Ir+Is+2√{square root over (IrIs)} cos(2πfift+Δθ)  (1)
The third term of the right side of the expression (1) indicates an alternating current electrical signal and the frequency fif thereof is equal to the frequency of beat caused from the interference between the reference light R and the signal light S. The frequency fif of an alternating current component of the heterodyne signal is called a beat frequency or the like. The first and second terms of the right side of the expression (1) indicate the direct current components of the heterodyne signal and correspond to a signal intensity of background light of interference light.
However, when the two-dimensional cross sectional image is obtained by the conventional low coherent interference method, it is necessary to scan the object to be measured 305 with a light beam and to successively detect reflection light waves from respective regions of the object to be measured 305 in a depth direction (z-direction) and a sectional direction (x-y plane direction). Therefore, the measurement of the object to be measured 305 is needed to be carried out in a wide range for instance by scanning the signal light and requires a long time. In addition, it is hard to shorten a measurement time in view of measurement fundamentals.
In views of such problems, an optical image measuring apparatus for shortening a measurement time has been proposed. FIG. 8 shows a fundamental structure of an example of such an apparatus. As shown in FIG. 8, an optical image measuring apparatus 400 includes a broad-band light source 401, a mirror 402, a beam splitter 403, a two-dimensional photo sensor array 404 serving as a photo detector, and lenses 406 and 407. A light beam emitted from the light source 401 is converted into a parallel light flux by the lenses 406 and 407 and a beam diameter thereof is widened thereby. Then, the parallel light flux is divided into two, that is, the reference light R and the signal light S by the beam splitter 403. The reference light R is subjected to Doppler frequency shift through z-scanning with the mirror 402. On the other hand, the signal light S is incident on the object to be measured 405 over a broad area of the x-y plane because the beam diameter is widened. Therefore, the signal light S becomes reflection light including information related to the surface and inner portion of the object to be measured 405 over a wide area. The reference light R and the signal light S are superimposed on each other by the beam splitter 403 and detected by elements (photo sensors) arranged in parallel on the two-dimensional photo sensor array 404. Thus, it is possible to obtain a two-dimensional cross sectional image of the object to be measured 405 in real time without light beam scanning.
An apparatus described in K. P. Chan, M. Yamada, and H. Inaba, “Electronics Letters”, Vol. 30, 1753 (1994) has been known as such a non-scanning type optical image measuring apparatus. In the apparatus described in the same document, a plurality of heterodyne signals outputted from a two-dimensional photo sensor array are inputted to signal processing systems arranged in parallel to detect the amplitude and phase of each of the heterodyne signals.
An optical image measuring apparatus is applied to measurement on an object to be measured which includes a birefringent layer such as a retinal nerve fibre layer (RNFL) of an eye (for example, see JP 2004-105708 A (claims 16, 32, and 49 to 56, specification paragraphs [0019], [0100] to [0108], and FIG. 11).
In the optical image measuring apparatus described in JP 2004-105708A, light from a light source is polarized in a longitudinal direction by a polarizer and allowed to enter a fiber. A ¼-wavelength delay plate (¼-wavelength plate) is provided on each of an optical path of signal light and an optical path of reference light. Interference light produced from the signal light and the reference light is separated into two polarized components orthogonal to each other by a polarization beam splitter to detect the respective polarized components. The detected two polarized components are combined with each other to obtain information related to the birefringent layer, thereby forming an image.
However, in order to obtain an image reflecting a birefringent property, of the object to be measured over a wide range using the optical image measuring apparatus described in JP 2004-105708 A, it is necessary to scan the object to be measured with the signal light as in the case using the apparatus shown in FIG. 7. Therefore, there is a problem in that a measurement time is prolonged.